Electrostatic fluid bed powder coating system

ABSTRACT

Charging apparatus for an electrostatic fluid bed powder coating system for uniformly coating a wire which is moving across the bed above the surface of fluidized powder, including a pair of elongated charging electrodes extending axially of the wire in the bed below the surface of the fluidized powder therein and a grounded control electrode located above the moving wire and having portions directly above and extending parallel to and substantially coextensive with each of the charging electrodes. The charging electrodes and control electrode portions are all located the same radial distance from the axis of the wire to be coated to form a cylindrical array having the wire to be coated as its axis.

Cited 1 Feb. 11, 1975 i 1 ELECTROSTATIC FLUID lBED POWDER COATING SYSTEM[75] Inventors: Roger W. Kellams, Anderson;

Clarence H. Dyer, Warren, both of [731 Assignee: General MotorsCorporation,

Detroit, Mich.

[221 Filed: Aug. 27, 1973 [21] Appl. No.: 391,578

1l/1970 Spiller et al 118/629 2/1972 .leromin et al. 118/629 PrimaryExaminerMervin Stein Assistant ExaminerLeo Millstein Attorney, Agent, orFirm-William S. Pettigrew [57] ABSTRACT Charging apparatus for anelectrostatic fluid bed powder coating system for uniformly coating awire which is moving across the bed above the surface of fluidizedpowder, including a pair of elongated charging electrodes extendingaxially of the wire in the bed below the surface of the fluidized powdertherein and a grounded control electrode located above the moving wireand having portions directly above and extending parallel to andsubstantially coextensive with each of the charging electrodes. Thecharging electrodes and control electrode portions are all located thesame radial distance from the axis of the wire to be coated to form acylindrical array having the wire to be coated as its axis.

4 Claims, 3 Drawing Figures SUPPLY PATENTED FEB] 1 i975 POWER SUPPLYELECTROSTATIC FLUID BED POWDER COATING SYSTEM This invention relates toan electrostatic fluid bed powder coating system, and more particularlyto charging apparatus for an electrostatic fluid bed powder coatingsystem for coating wire.

One feature of the invention is that it provides an improvedelectrostatic fluid bed powder coating system. Another feature of theinvention is that it provides charging apparatus so constructed andarranged as to provide a uniform coating of powder on an elongatedworkpiece, as a wire, which is moved axially across the fluid bed andabove the surface of fluidized powder therein. Still a further featureof the invention is that it utilizes elongated charging and controlelectrodes extending axially of the elongated workpiece and arrangedsymmetrically about its axis, all of the electrodes being radiallyequidistant from said axis.

Other features and advantages of the invention will be apparent from thefollowing description and from the drawings, in which:

FIG. 1 is a transverse vertical section through a fluid bed powdercoating system incorporating the invention, some of the structure beingshown schematically;

FIG. 2 is a longitudinal vertical section through the system, beingtaken along the line 2-2 of FIG. 1, and

FIG. 3 is a top plan view taken along the line 33 of FIG. 2.

Electrostatic fluid bed powder coating systems are well known in theart. In some arrangements, the workpiece to be coated is dipped into thefluidized powder which is charged electrostatically so that particles ofthe powder adhere to the workpiece, which generally is heated. In otherarrangements, and particularly arrangements designed to coat elongatedworkpieces, as a wire of indeterminate length, the workpiece is movedhorizontally across the fluid bed above the surface of fluidized powdertherein. The electric charge imparted to the fluidized powder causespowder particles to be attracted to the workpiece, which is generallygrounded. Patents illustrative of this type of electrostatic fluid bedpowder coating system are Barford et al. U.S. Pat. No. 3,248,253, PointU.S. Pat. No. 3,336,903, and Beebe et al. U.S. Pat. No. 3,396,699. For ageneral discussion of fluid bed coating, reference may be had to GemmerU.S. Pat. No. 2,844,489.

While, as noted above, electrostatic fluid bed powder coating systemsare old, difficulties have been experienced in providing a coating ofuniform thickness, particularly on wire of indeterminate length which ismoved across the bed above the surface of fluidized powder therein. Thisinvention provides improved charging apparatus resulting in more uniformpowder coating on the moving wire.

Referring now more particularly to the drawings, is a conventionalfluidized bed container of nonconductive material having a bottom 10aopposite side walls 10b and end walls 10c. Spaced above the bottom 10aof the fluidized bed container is a porous plate 12 which, as discussedfully in Gemmer U.S. Pat. No. 2,448,489 permits passage of air or othergas under pressure. Powder 14 in the container above the porous plate 12is fluidized by the passage of the air as is fully discussed in GemmerU.S. Pat. No. 2,844,489. Fluidizing gas, which may be air, may besupplied by a conventional air compressor 16 connected by a hose 18 tothe chamber 10d in the container 10 between the bottom 10a and theporous plate 12.

Apertures T9 in the end walls are provided for the passage of anelongated workpiece 20 here shown as a wire of indeterminate lengthwhich is moved in the direction of the arrows (FIGS. 2 and 3) byconventional apparatus (not shown) horizontally and axially across thefluid bed above the surface of fluidized powder therein. As illustratedschematically in FIG. 2, the wire 20 is electrically grounded.

The charging apparatus comprising the invention includes one or morepairs of elongated charging electrodes in the fluid bed below thesurface of the fluidized powder therein and a grounded control electrodelocated above the workpiece. These electrodes are arranged symmetricallywith relation to the wire 20 in a manner to be described to provide auniform powder coating on the wire as it moves across the fluid bed.

An outer pair of elongated charging bar electrodes 22a and 22b arepositioned in the fluid bed below the surface of the fluidized powdertherein. They are supported in the bed by insulating supports 23. Theelectrodes 22a and 22b are formed of electrically conducting material,preferably copper, and are round in transverse section. They preferablyhave rounded ends as shown in FIGS. 2 and 3. The electrodes 22a and 22bextend parallel to the axis of the wire 20 and are located symmetricallyon opposite sides of the wire and are radially equidistant from the axisof the wire.

While not necessary in certain embodiments of the invention, in theembodiment illustrated, there is an inner pair of charging barelectrodes 24a and 24b in the fluid bed between and extending parallelto the outer electrodes 22a and 22b. The electrodes 24a and 24b may alsobe mounted on the insulating supports 23. Each inner electrode 240 and24b is substantially the same size and shape as the outer electrodes andeach inner electrode is spaced the same distance from its adjacent outerelectrode as the other inner electrode is spaced from its adjacent outerelectrode. The inner electrodes are located the same :radial distancefrom the axis of the wire 20 as are the outer charging electrodes.Additional pairs of electrodes may be used if desired.

A power supply 26 has one terminal (preferably the positive terminal)grounded and the other terminal connected by a cable 28 to each of theelectrodes 22a, 22b, 24a and 24b. In order to provide equal voltages onall of the charging electrodes, it is preferred that they be connectedin parallel to the power supply.

A control electrode of smaller diameter than the charging electrodes islocated above the wire 20. The control electrode, which is grounded, maybe formed of a rectangular wire 30 supported in the container 10 byinsulators (not shown) and having portions 30a and 30b directly aboveand extending parallel to and substantially coextensive with each of theouter charging electrodes 22a and 22b. The electrode 30 is supported onthe walls of cabinet 10 by brackets 31. Separate grounded wire portions30a, 30b may be used if desired. The portions 30a and 30b of the controlelectrode are located the same radial distance from the axis of the wire20 as are the charging electrodes 22a, 22b, 24a and 24b. The arrangementthus provides a charging system in which the charging electrodes andcontrol electrodes are arranged in a cylindrical manner with the wire tobe coated as the axis of the cylinder.

The number of electrodes needed, their size, spacing and other values inthe system, are determined by a number of parameters. These parametersinclude the diameter, electrical conductivity and the rate of movementof the wire to be coated; the type of powder to be applied, and thecharging voltage which is used. In one system which has been devised,the wire 20 was copper wire having a diameter of 0.098 inches and wasmoving across the bed at a speed of ten feet per minute. The coatingmaterial 14 was an epoxy powder which was coated on the wire 20 to athickness of 0.008 inches. The power supply 26 provided kilovolts(negative). The dimensions of the parts of the charging system were asfollows:

a. Diameter of charging electrodes 22a, 22b 24a and 24b 0.5 inches b.Length of charging electrodes 22a, 22b, 24a and 24b 24 inches c. Radialdistance between wire 20 and electrodes 22, 24 and 4.5 inches d.Distance between outer electrodes 22a and 22b 6 inches e. Angle includedbetween radial lines drawn between electrode 22a and wire 20 andelectrode 24a and wire 20 18 f. Diameter of control electrode 30 0.036inches g. Vertical distance between electrodes 22a, 22b and the surfaceof fluidized powder 14 0.5 inches.

The system described above provided a powder coating of uniformthickness of 0.008 inches on the wire 20.

After application of the powder, the wire may pass through an oven wherethe wire is heated and the coating is fused thereon as described inBeebe et al. US. Pat. No. 3,396,699.

While I have shown and described one embodiment of my invention, it iscapable of various modifications. Changes, therefore, may be made in theconstruction and arrangement without departing from the spirit and scopeof the invention as set forth in the appended claims.

What is claimed is as follows:

1. In an electrostatic fluid bed powder coating system having a powderfluidizing bed, a source of fluidizing gas therefor, and means formoving an elongated workpiece horizontally and axially across said bedabove the surface of fluidized powder therein, charging apparatuscomprising:

a voltage source;

a pair of elongated charging electrodes in said bed below the surface ofthe fluidized powder therein, said electrodes extending parallel to theaxis of said workpiece and being located symmetrically on opposite sidesof said axis and radially equidistant therefrom;

means connecting said electrodes to said voltage source;

a control electrode located above said workpiece and having portionsdirectly above and extending parallel to and substantially coextensivewith each of said charging electrodes, said portions being located thesame radial distance from the axis of said workpiece as are saidcharging electrodes;

means connecting said control electrode portions and said workpiece to avoltage different from that of the charging electrodes.

2. In an electrostatic fluid bed powder coating system having a powderfluidizing bed, a source of fluidizing gas therefor, and means formoving a wire horizontally and axially across said bed above the surfaceof fluidized powder therein, charging apparatus comprising:

a voltage source;

a pair of similar elongated charging bar electrodes in said bed belowthe surface of the fluidized powder therein, said electrodes being roundin transverse section and having rounded ends, and said electrodesextending parallel to the axis of said wire and being locatedsymmetrically on opposite sides of said wire and radially equidistantfrom the axis thereof;

means connecting said electrodes to said voltage source;

a control electrode of smaller diameter than said charging electrodeslocated above said wire and having portions directly above and extendingparallel to and substantially coextensive with each of said chargingelectrodes, said portions being located the same radial distance fromthe axis of said wire as are said charging electrodes;

and means connecting said control electrode portions and said wire toground.

3. In an electrostatic fluid bed powder coating system having a powderfluidizing bed, a source of fluidizing gas therefor, and means formoving an elongated workpiece horizontally and axially across said bedabove the surface of fluidized powder therein, charging apparatuscomprising:

a voltage source;

an outer pair of elongated charging electrodes in said bed below thesurface of the fluidized powder therein, said electrodes extendingparallel to the axis of said workpiece and being located symmetricallyon opposite sides of said axis and radially equidistant therefrom;

an inner pair of elongated charging electrodes in said bed between andextending parallel to said outer electrodes, each one of said innerelectrodes being spaced the same distance from its adjacent outerelectrode as the other inner electrode is spaced from its adjacent outerelectrode, said inner electrodes being located the same radial distancefrom the axis of said workpiece as are said outer charging electrodes;

means connecting said outer and inner electrodes to said voltage source;

a control electrode located above said workpiece and having portionsdirectly above and extending parallel to and substantially coextensivewith each of said outer charging electrodes, said portions being locatedthe same radial distance from the axis of said workpiece as are saidouter charging electrodes;

and means connecting said control electrode portions and said workpieceto a voltage different from that of the charging electrodes.

4. In an electrostatic fluid bed powder coating system having a powderfluidizing bed, a source of fluidizing gas therefor, and means formoving a wire horizontally and axially across said bed above the surfaceof fluidized powder therein, charging apparatus comprising:

a voltage source;

an outer pair of elongated charging bar electrodes in said bed below thesurface of the fluidized powder therein, said electrodes being round intransverse section and having rounded ends, and said electrodesextending parallel to the axis of said wire and being locatedsymmetrically on opposite sides of said wire and radially equidistantfrom the axis thereof;

an inner pair of elongated charging bar electrodes in said bed betweenand extending parallel to said outer electrodes, each inner electrodebeing substantially the same size and shape as said outer electrodes andeach being spaced the same distance from its adjacent outer electrode asthe other inner electrode is spaced from its adjacent outer electrode,and said inner electrodes being located the same radial distance fromthe axis of said wire tions and said wire to ground.

r: m it :It:

1. In an electrostatic fluid bed powder coating system having a powderfluidizing bed, a source of fluidizing gas therefor, and means formoving an elongated workpiece horizontally and axially across said bedabove the surface of fluidized powder therein, charging apparatuscomprising: a voltage source; a pair of elongated charging electrodes insaid bed below the surface of the fluidized powder therein, saidelectrodes extending parallel to the axis of said workpiece and beinglocated symmetrically on opposite sides of said axis and radiallyequidistant therefrom; means connecting said electrodes to said voltagesource; a control electrode located above said workpiece and havingportions directly above and extending parallel to and substantiallycoextensive with each of said charging electrodes, said portions beinglocated the same radial distance from the axis of said workpiece as aresaid charging electrodes; means connecting said control electrodeportions and said workpiece to a voltage different from that of thecharging electrodes.
 2. In an electrostatic fluid bed powder coatingsystem having a powder fluidizing bed, a source of fluidizing gastherefor, and means for moving a wire horizontally and axially acrosssaid bed above the surface of fluidized powder therein, chargingapparatus comprising: a voltage source; a pair of similar elongatedcharging bar electrodes in said bed below the surface of the fluidizedpowder therein, said electrodes being round in transverse section andhaving rounded ends, and said electrodes extending parallel to the axisof said wire and being located symmetrically on opposite sides of saidwire and radially equidistant from the axis thereof; means connectingsaid electrodes to said voltage source; a control electrode of smallerdiameter than said charging electrodes located above said wire andhaving portions directly above and extending parallel to andsubstantially coextensive with each of said charging electrodes, saidportions being located the same radial distance from the axis of saidwire as are said charging electrodes; and means connecting said controlelectrode portions and said wire to ground.
 3. In an electrostatic fluidbed powder coating system having a powder fluidizing bed, a source offluidizing gas therefor, and means for moving an elongated workpiecehorizontally and axially across said bed above the surface of fluidizedpowder therein, charging apparatus comprising: a voltage source; anouter pair of elongated charging electrodes in said bed below thesurface of the fluidized powder therein, said electrodes extendingparallel to the axis of said workpiece and being located symmetricallyon opposite sides of said axis and radially equidistant therefrom; aninner pair of elongated charging electrodes in said bed between andextending parallel to said outer electrodes, each one of said innerelectrodes being spaced the same distance from its adjacent outerelectrode as the other inner electrode is spaced from its adjacent outerelectrode, said inner electrodes being located the same radial distancefrom the axis of said workpiece as are said outer charging electrodes;means connecting said outer and inner electrodes to said voltage source;a control electrode located above said workpiece and having portionsdirectly above and extending parallel to and substantially coextensivewith each of said outer charging electrodes, said portions being locatedthe same radial distance from the axis of said workpiece as are saidouter charging electrodes; and means connecting said control electrodeportions and said workpiece to a voltage different from that of thecharging electrodes.
 4. In an electrostatic fluid bed powder coatingsystem having a powder fluidizing bed, a source of fluidizing gastherefor, and means for moving a wire horizontally and axially acrosssaid bed above the surface of fluidized powder therein, chargingapparatus comprising: a voltage source; an outer pair of elongatedcharging bar electrodes in said bed below the surface of the fluidizedpowder therein, said electrodes being round in transverse section andhaving rounded ends, and said electrodes extending parallel to the axisof said wire and being located symmetrically on opposite sides of saidwire and radially equidistant from the axis thereof; an inner pair ofelongated charging bar electrodes in said bed between and extendingparallel to said outer electrodes, each inner electrode beingsubstantially the same size and shape as said outer electrodes and eachbeing spaced the same distance from its adjacent outer electrode as theother inner electrode is spaced from its adjacent outer electrode, andsaid inner electrodes being located the same radial distance from theaxis of said wire as are the outer charging electrodes; means connectingsaid inner and outer electrodes to said voltage source; a controlelectrode of smaller diameter than said charging electrodes locatedabove said wire and having portions directly above and extendingparallel to and substantially coextensive with each of said outercharging electrodes, said portions being located the same radialdistance from the axis of said wire as are said outer chargingelectrodes; and means connecting said control electrode portions andsaid wire To ground.